Coupling of open reading frames by translational bypassing

Translational bypassing joins the information found within two disparate op en reading frames into a single polypeptide chain. The underlying mechanism centers on the decoding properties of peptidyl-transfer RNA (tRNA) and inv olves three stages: take-off, scanning, and landing. In take-off, the pepti dyl-tRNA/messenger RNA (mRNA) complex in the P site of the ribosome dissoci ates, and the mRNA begins to move through the ribosome. In scanning, the pe ptidyl-tRNA probes the mRNA sliding through the decoding center. In landing , the peptidyl-tRNA re-pairs with a codon with which it can form a stable i nteraction. Although few examples of genes are known that rely on translati onal bypassing to couple open reading frames, ribosomes appear to have an i nnate capacity for bypassing. This suggests that the strategy of translatio nal bypassing may be more common than presently appreciated. The best chara cterized example of this phenomenon is T4 gene 60, in which a complex set o f signals stimulates bypassing of 50 nucleotides between the two open readi ng frames. In this review, we focus on the bypassing mechanism of gene 60 i n terms of take-off, scanning, and landing.